[pull] master from deepmodeling:master - #312

## Summary

- document TensorFlow 2 and PyTorch-Exportable backend selection,
training, freezing, compression, formats, and inference routing;
- fold the JAX training and Hessian-freeze documentation from #5930 into
this PR;
- update model and descriptor backend icons after checking the current
TF2, PyTorch-Exportable, and JAX registrations;
- distinguish JAX non-energy fitting/model availability from its current
energy-only training loss support;
- document the DP and JAX compression routes requested by #5923.

Closes #5919.
Closes #5920.
Closes #5923.
Supersedes #5930.

## Validation

- `ruff format .`
- `ruff check .`
- `uvx pre-commit run mdformat --files ...` for all changed Markdown
files
- `dp --dp compress --help`
- `dp --jax compress --help`
- `git diff --check`

Coding agent: Codex
Codex version: codex-cli 0.144.6
Model: gpt-5.6-sol
Reasoning effort: xhigh


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Expanded backend guidance for training, freezing, compression,
conversion, and inference, including TensorFlow 2, PyTorch-Exportable,
JAX, Paddle, and DP.
* Added backend selection commands, workflow examples, multi-task
checkpoint selection, and Hessian-enabled JAX freezing.
* Documented backend-specific file formats, export artifacts, ABI
selection, optional TensorFlow 2 JIT compilation, and feature
limitations.
* Updated model pages with corrected backend support, spin and
graph-native inference guidance, and standardized command syntax.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@ustc.edu.cn>
Co-authored-by: njzjz-bot <njzjz.bot@gmail.com>
Co-authored-by: njzjz-bot <njzjz-bot@users.noreply.github.com>